This invention relates to a galvanic sensor of the solid electrolyte type that is responsive to oxygen content in an exhaust gas. It more particularly relates to improvements in exhaust gas flow over a disc of solid electrolyte in an internal combustion engine exhaust gas sensor.
The solid electrolyte in galvanic exhaust gas oxygen responsive sensors must be heated to an elevated temperature for the sensor to be opertive. At higher temperatures, the solid electrolyte exhibits high ionic conductivity. The earlier U.S. Ser. No. 787,900 Howarth, filed Apr. 15, 1977, now U.S. Pat. No. 4,129,099 and entitled "Galvanic Exhaust Gas Sensor," discloses that the sensor can be heated by the exhaust gas or by a resistance type heater. It also cites earlier work in maintaining the electrolyte at a constant operating temperature. Further, it discloses an electrolyte dopant for making the sensor output voltage insensitive to temperature variations above a predetermined temperature.
The sensors referred to above include an oxygen conductive solid electrolyte body, usually of partially or fully stabilized zirconia, having negligible electron conductivity. Thin porous platinum electrodes on opposite faces of the body respectively serve as a reference and exhaust gas electrode. It is customary to prevent direct impingement of the exhaust gases on the zirconia element, so as not to erode the thin platinum electrode. This also slows the exhaust gases down somewhat so that they can be more fully equilibrated at the electrode. A variety of shield configurations and porous electrode coatings have already been proposed to provide improved exhaust gas flow over the exhaust gas electrode.
I have recognized that the exhaust gas stream not only varies in composition but also in its flow. It changes in pressure and velocity, as well as in temperature. These variations may induce changes in sensor output voltage, even though oxygen concentration in the exhaust gas remains substantially constant. Such variations can even completely mask subtle changes in oxygen content of the exhaust gases.
This invention involves a disc-like electrolyte body in a unique housing assembly. The assembly includes a shield over the exhaust gas electrode for stabilizing exhaust gas flow over that electrode. With the more stable exhaust gas flow, the exhaust gas electrode can be more sensitive to variations in oxygen content in the exhaust gas. If flow is stabilized, constant temperature of a heated sensor can be more readily maintained, particularly if the solid electrolyte is isolated from thermal conduction with its housing. In a preferred example, the shield increases exhaust gas flow over the exhaust electrode at low exhaust gas stream flows, and limits flow over that electrode at higher stream flows. With a more stable exhaust gas velocity and pressure at the exhaust electrode, the electrode is more responsive to variations in oxygen content in the exhaust gas. More constant but lower level flows over the electrode may even make the sensor less sensitive to momentary temperature fluctuations in exhaust gas temperature. Preferably, flow through the sensor is generally constant at all exhaust gas stream flows appreciably above the lowest stream flow normally occurring during engine operation, such as at engine idle.